1. Field of the Invention
This invention relates to subsea, driven-pile anchor systems. More particularly, it relates to subsea piles used to anchor the tendons of tension leg platforms or other vertically-moored structures to the seafloor.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
A Tension-Leg Platform or TLP is a vertically moored floating structure normally used for the offshore production of oil or gas, and is particularly suited for water depths greater than 300 meters (about 1000 ft). The design has also been proposed for wind turbines.
The platform is permanently moored by means of tethers or tendons grouped at each of the structure's corners. The tendons have relatively high axial stiffness (low elasticity) such that virtually all vertical motion of the platform is eliminated. This feature allows the platform to have the production wellheads on deck (connected directly to the subsea wells by rigid risers), instead of on the seafloor. This makes for a less expensive well completion and provides better control over the production from the oil or gas reservoir.
Conventionally, the tendons of a TLP are secured to pile foundations comprising tubular piles driven into the seafloor by mechanical hammers. Such anchoring means provides high resistance to the tensile load applied by the floating TLP through the tendons.
U.S. Pat. No. 5,551,804 discloses a method of driving a pile wherein a pile is plugged at its lower end or tip region so as to make it easier to drive. Additionally, such a plugged pile facilitates the transportation and handling of the pile prior to its driving. This invention comprises the use of a plug at or near the bottom or end of a pile during handling, assembly, and lowering of the pile to self support in the sea bottom, followed by the driving of the pile with the plug intact. General consensus in the industry is that the plug will make the driving of the pile more difficult since it presents more of a profile that must be moved through the soil. However, this was not found to be the case in highly sensitive clays; the driving of a plugged pile is considerably easier than the driving of an open pile. While plugs have been employed in the past to facilitate certain aspects of pile transportation, handling, assembly, and lowering, they were always removed prior to pile driving because of the belief that the plug would make the pile more difficult or impossible to drive. Previously, the advantages derived from using plugs did not compensate for the costs related to installing and removing the plugs.
U.S. Pat. Nos. 6,318,933 and 6,142,709 describe a foundation system for tension leg platforms without a foundation template, wherein each tendon is directly connected to a socket inside the pile, the piles being positioned for driving purpose by means of a pile-driving template which is employed as a spacing device is described. The pile-driving template is positioned with the aid of pins that slot into guides built into the well template. After the groups of piles needed to anchor a corner of the platform have been driven in, the pile-driving template is withdrawn and repositioned so as to enable the piles for the other group of legs to be driven; this process continues until all of the pile-driving is finished. Alternatively one single pile-driving template may be employed to guide the driving of all the piles thus doing away with the need to reposition the template every time. The bottom ends of the piles are conical in shape, and after the piles have been driven they are filled with a high specific gravity material.
U.S. Pat. No. 3,984,991 describes an anchor which includes a tubular body, a top closure and a bottom closure secured to opposite ends of the tubular body, a plurality of drilling cutters mounted on the bottom closure, a neck having an external groove therein secured to the top closure, an opening in both the top and bottom closures, means for co-acting with a drill string extending into the closures for sealing to maintain the interior of the body substantially free of water when submerged, a ratchet collar adapted to co-act with a mating ratchet collar of the drill string for rotating the anchor to cause it to drill into the bottom of a body of water, a swivel adapted to be lowered onto the neck of the top closure when it has been set, latching dogs engaging in the external groove to secure the swivel to the body, and floatation means for signaling the surface that the swivel is set. The method of setting an anchor assembly including the steps of lowering an anchor body having cutters on the bottom thereof on the end of a drill string extending through the top of the anchor body and into the bottom for circulation of drilling fluid onto the face of the formation being drilled, the drill string being sealed to the anchor body to prevent entry of water therein, rotating the drill string and anchor body to drill the hole and lower the anchor body into the hole simultaneously, cementing around the exterior of the anchor body, then cementing the interior of the anchor body, lowering a swivel onto the top of the anchor body and signaling the seating of the swivel.
U.S. Pat. No. 5,582,491 describes a system to increase the tension capacity of pipe piles driven into the ocean floor. A pile cap is attached to a pipe pile. A partition is installed below the pile cap creating an air chamber between them that is at surface atmospheric air pressure. An external conduit containing a valve that is closed connects the pile's interiors above and below the partition. The pile is driven into the ocean floor filled with entrapped sea water below the partition so that little or no soil core is generated. After the clay soils adjacent to the pile have regained their strength, the valve is opened. A small amount of sea water expands into the air chamber. The pressure on both sides of the partition and the bottom of the pile cap is now slightly above surface atmospheric air pressure. It is the that the downward force of hydrostatic pressure on top of the pile cap increases the tension capacity of the driven pile.
U.S. Pat. No. 6,536,993 describes an apparatus for providing a mooring anchorage and a method of drilling and installing a pile in ground comprising the steps of: providing a pile, providing a drill bit at an end of the pile rotatable relative to the pile, engaging the ground with the drill bit, and rotating the drill bit relative to the ground and the pile generating a hole into which the pile is received.
U.S. Pat. No. 6,312,195 describes a method of installing a foundation for a tension leg platform that eliminates the foundation template as a permanent, load bearing part of the foundation. Piles are installed by, for example, being driven into the ocean floor so that each pile is secured to the ocean floor, but is unsecured to any other structure that is on the ocean floor. A tension leg platform is coupled via tendon structures to the piles so that anchoring load paths are defined from the tension leg platform to the ocean floor in a plurality of generally vertical paths extending in axial alignment through the tendon structures to the pile and the ocean floor. Each of the tendon structure to pile anchoring systems is said to be substantially independent of one another.
U.S. Pat. No. 5,020,764 describes a pole ballasting device adapted to be positioned about the lower end of one or more poles of a pole-using temporary structure for the purpose of holding the poles. The device includes at least two discrete blocks having, in their assembled condition, one or more common vertically extending through holes for receiving the poles therethrough. The blocks are arranged along planes passing through the through holes. A joining mechanism is provided for separably joining the blocks into a unit. A container is formed in at least one of the blocks and adapted to contain a fluidic load therein.